German 33 15 085 C2 describes a method for zero-setting a thermal conductivity detector (TCD) in an incubator cell, in which an atmosphere containing CO.sub.2 is contained. The atmosphere can be controlled with the TCD. The measuring side, of measuring cell, or the TCD system continuously has atmosphere from the chamber applied thereto, and the measured value can be compared with a reference gas, typically ambient air.
The type of incubator chamber is known, see, for example U.S. Pat. No. 4,336,329, Hesse et al., the disclosure of which is hereby incorporated by reference.
The thermal conductivity cells have a reference side. During a measuring phase, air or some other external gas is supplied as a reference gas. During a calibrating phase, this supply is interrupted, and the reference side is exposed to the atmosphere within the chamber. The signal which is so obtained is stored, and in a measuring phase subtracted from the measuring signal desired from the measuring side of the TCD, in order to obtain a zero-offset correction signal. This correction signal is used until the next subsequent calibration phase at which time, the previously obtained correction signal is replaced by a new offset correction signal.
It has been found that this system requires a comparatively complex construction for the chamber, typically the incubator, since the reference side of the TCD must be placed in communication with the interior of the chamber, as well as with a reference gas, for example ambient external atmosphere. These communications must be absolutely separate and completely air-tight in operation. The measuring side, as well as the reference side, must never be exposed to the same atmosphere during the measuring phase. Further, the calibration is not an absolute zero-offset calibration of the entire cell, but, rather, provides a signal to correct drift of the actual sensor cells of the thermal conductivity sensor, or thermal conductivity sensor system.
German 21 33 119 B2, Boirat et al., describes an incubator having a jacket of thermal material, in which the horizontal walls of the jacket are formed of two heating plates. The upper plate is held at a controlled temperature, whereas the lower plate remains at a lower temperature. The resulting temperature gradient provides an intermediate temperature for introduced cultures of bacteria. This permits simultaneous loading of the chamber and automatic control of bacteria cultures. A plurality of controlled heating plates must be used, so that the construction is complex and expensive.